1. Field of the Invention
This invention relates to a dry test strip for measuring the concentration of an analyte in a biological fluid; more particularly, a test strip that calorimetrically measures the concentration of glucose in whole blood.
2. Description of the Related Art
Many visual test devices have been developed for measuring the concentration of certain analytes in biological fluids. These devices have, for example, measured glucose, cholesterol, proteins, ketones, phenylalanine, or enzymes in blood, urine, or saliva.
Dry phase reagent strips incorporating enzyme-based compositions are used extensively in hospitals, clinical laboratories, physician's offices, and homes to test samples of biological fluids for glucose concentration. In fact, reagent strips have become an everyday necessity for many of the nation's several million diabetics. Since diabetes can cause dangerous anomalies in blood chemistry, it can contribute to vision loss, kidney failure, and other serious medical consequences. To minimize the risk of these consequences, current teaching counsels persons with diabetes to measure their blood glucose level from two to seven times a day, depending on the nature and severity of their individual cases. Based on the observed pattern in the measured glucose levels, the patient and physician together make adjustments in diet, exercise and insulin intake to better manage the disease. Clearly, this information should be available to the patient immediately, through the use of a simple-to-use meter and strip system that is rapid, inexpensive, and accurate.
Reagent strips are known that contain an indicator which turns a different shade of color, depending on the concentration of glucose in a biological fluid that has been applied to the strip. Although some of these strips use reduction chemistries, more commonly they involve an oxidizable dye or dye couple. Some of the strips include an enzyme, such as glucose oxidase, which is capable of oxidizing glucose to gluconic acid and hydrogen peroxide. They also contain an oxidizable dye and a substance having peroxidative activity, which is capable of selectively catalyzing oxidation of the oxidizable dye in the presence of hydrogen peroxide.
U.S. Pat. No. 4,935,346, issued Jun. 19, 1990 to R. Phillips et al., discloses a meter, strip, and method for determining the glucose concentration in a sample of whole blood (see also U.S. Pat. No. 5,304,468). The method involves simply applying a sample of whole blood to a first ("sample") surface of an inert porous matrix that is impregnated with a reagent. The sample migrates toward the opposite, "testing" surface, as the glucose interacts with the reagent to produce a light-absorbing reaction product. A reading of reflectance from the testing surface indicates the glucose concentration. Reflectance measurements are made at two separate wavelengths in order to eliminate interferences. A timing circuit is triggered by an initial decrease in reflectance caused by wetting of the testing surface by the sample having passed through the matrix.
U.S. Pat. No. 5,306,623, issued Apr. 26, 1994 to Kiser et al., discloses a visual blood glucose test strip that involves applying a glucose-containing whole blood sample to one side of the strip and taking the glucose reading on the opposite side, after red blood cells have been separated out and the sample has reacted with a reagent in the strip. An anisotropic polysulfone membrane was found especially useful as a single layer matrix for the strip.
U.S. Pat. No. 5,453,360, issued Sep. 26, 1995 to Y. S. Yu, discloses a dye couple useful in dry reagent strips for detecting analytes, such as glucose, in biological fluids. The dye couple comprises 3-methyl-2-benzothiazolinone hydrazone and 8-anilino-1-naphthalenesulfonate and is used as an indicator in a reaction cascade producing a strong oxidizing agent, such as hydrogen peroxide. An advantage of the couple is that it is soluble in aqueous solution, but becomes insoluble upon oxidative coupling, thereby minimizing fading and providing a stable endpoint.
A meter that has come into widespread use for self-monitoring of blood glucose is the One Touch.RTM. II meter, which uses a strip that is described, inter alia, in U.S. Pat. Nos. 4,935,346 and 5,304,468, discussed above. The meter and strip permit a user to measure glucose concentration in a whole blood sample quickly, easily, and accurately. The sample is applied to one surface of the strip and the measurement made on the opposite surface. A portion of the whole blood sample penetrates from the sample surface to the testing surface, and the blood color can be observed from the testing surface.